1. Field of the Invention
The invention relates to magnetic compasses used in aviation, in general, and to improved magnetic compasses which correct for acceleration and turning errors as well as magnetic dip forces, in particular.
2. Prior Art
A magnetic compass was one of the first instruments to be installed in an airplane, and it is still the only direction-seeking instrument in many smaller aircraft. One great advantage of such a compass is that it is a self-contained unit which does not require electrical or vacuum power. To determine direction, the compass uses a simple bar magnet with two poles. The bar magnet in the compass is mounted so that it can rotate freely and align itself automatically with the Earth""s magnetic field.
However, magnetic compasses are subject to a problem called xe2x80x9cmagnetic dipxe2x80x9d because the lines of flux of the Earth""s magnetic field are not parallel to the Earth""s surface (except at the magnetic equator). Since the compass needle aligns itself with the lines of flux, the north-seeking end of the needle tends to dip toward the Earth (in the Northern Hemisphere). This dip angle is caused by the vertical component of the Earth""s magnetic field and increases from zero at the magnetic equator to almost 90 degrees near the magnetic pole. Many magnetic compasses (including those used in aviation) have a small weight on the south-pointing (again in the Northern Hemisphere) end of the needle to counteract this magnetic dip by using gravitational force. That is, a counterweight is added to the end of the compass needle which points away from the magnetic pole. This arrangement does, indeed, almost completely fix the dip problem and has been used on compasses for many years. Unfortunately, this technique introduces two new errors, called the xe2x80x9cacceleration errorxe2x80x9d and the xe2x80x9cturning error.xe2x80x9d These errors are exhibited as unwanted needle movements when the compass is subjected to inertial forces, viz. due to change in speed and/or direction. The inertial force acts on the counterweight of the needle and produces a torque about the pivot at which the compass is suspended because the mass of the needle is not equal on both sides of the pivot point. This torque causes the needle and the attached card (in aviation compasses) to rotate and thereby give a false reading.
During acceleration or deceleration of an airplane on an easterly or westerly heading, an erroneous indication will occur. During acceleration, inertia causes the compass weight on the south end of the bar magnet to lag slightly and turn the compass toward a North indication even though no change of direction has taken place. During a deceleration, inertia causes the weight to move slightly ahead, which moves the compass toward a southerly heading indication. The compass will return to its previous, and proper, heading once the acceleration or deceleration subsides.
This acceleration error does not occur when flying on a directly north or south heading because the dip compensation weight is in line with the direction of travel, but becomes more pronounced as the plane""s heading is closer to due east or west. These acceleration error examples are valid only for the northern hemisphere; the effects are reversed in the southern hemisphere.
Turning error is also caused by inertial forces acting on the counterweight. It is most pronounced when turning from headings of due north or south. At the beginning of a turn from a heading of north, inertia (in the form of centrifugal force) forces the counterweight to the outside of the turn so the compass initially indicates a turn in the opposite direction. When the turn is established, the compass card begins to rotate in the correct direction, but it lags behind the actual heading. The amount of lag decreases as the turn continues, then disappears as the airplane reaches a heading of east or west.
When turning from a heading of east or west to a heading of north, there is no error at the beginning of the turn. However, as the heading approaches north, the compass increasingly lags behind the airplane""s actual heading. When turning from a heading of south, the compass initially indicates a turn in the proper direction but leads the airplane""s actual heading This error also diminishes as the airplane reaches a heading of east or west. Turning from east or west to a heading of south causes the compass to move correctly at the start of a turn, but then it increasingly leads the actual heading as the airplane nears a southerly direction. (As in acceleration errors, these turning errors are only valid for flight in the Northern Hemisphere but in the Southern Hemisphere act in the opposite directions.) A more detailed description may be found in xe2x80x9cPrivate Pilot Manualxe2x80x9d by Jeppeson Sanderson Training Products.
One solution to the error situation in a magnetic compass is to balance the inertial forces on each end of the needle without affecting the weight distribution. This is accomplished by fastening a small capsule to the north-seeking end of the needle which capsule has the same mass as the counterweight, but has zero weight because it is designed to have the same density as the fluid with which the compass is filled. Of course, the capsule could be designed to be integral with the needle, as well.
In a preferred embodiment, the capsule is hollow. The result of incorporating such a capsule is that inertial forces cause no torque because the mass of the needle is equal on both sides of the pivot. The balance of the gravitational force acting on the counterweight versus the vertical component of the magnetic force acting on the needle remains undisturbed.
The magnetic compass is made more usable and less error prone over a wide range of acceleration and turning conditions. It may, therefore, be used as a stable reference source for a heading Indicator or other instrumentation.
The inherent reliability of the magnetic compass is retained inasmuch as no moving parts nor a power source are required.
The improved magnetic compass is considerably more useful to a pilot because its reading is more stable and accurate. Since it is not subject to the acceleration and turning errors, it can be read in non-straight-and-level flight maneuvering. This difference could be of extreme importance should the aircraft experience a vacuum failure (which renders the gyroscopic Heading Indictor inoperable in a typical General Aviation craft).